This invention relates to joining two preformed metal panels together to form a closure panel and more particularly to hemming closure panels made from aluminum material.
It is known in the automotive industry to join two metal preformed panels into a unitary hollow structural unit. Typical units of this type include vehicle doors, hoods, and trunk lids. Collectively, these units are referred to as closure panels.
A conventional process of joining the two panels together is referred to as hemming and results in a flange of the outer panel being folded over and gripping the edge of the inner panel. The process is well known.
Typical vehicle closure panels are made of steel. Nested inner and outer panels are hemmed to form a flat hem that connects the panels. These panels have desirable strength and impact absorbing properties.
The automobile industry has substituted materials that are lighter than steel materials where substitution is practical to reduce weight and thereby improve fuel economy. Aluminum, which has been applied in vehicle construction, has desirable strength and impact absorbing properties as well as a desirable weight vis-a-vis steel. However, the processes used to form a flat hem on nested steel panels have not been applied successfully on nested aluminum panels to form a flat hem.
The metallurgical characteristics of the aluminum material cause the outer panel in the area of the hem line or break line to crack during the hemming process. Furthermore, the peripheral edge of the hemmed outer panel springs back partially away from the inner panel, not forming the desired flat hem.
A need exists for a method that provides for the hemming of nested aluminum panels to a flat hem.
The present invention provides a hemmed aluminum closure panel and method for forming a flat hem on nested aluminum panels without cracking of the outer panel along the hem line.
More specifically, in accordance with an aspect of the invention, a method for forming a vehicle closure panel assembly from outer and inner metal preformed panels disposed between lower and upper tools, includes the steps of:
applying a resilient material between the inner and outer panels around the area of the hem line;
nesting said inner panel and said outer panel into said lower tool;
applying a clamp down force onto the inner panel to cause said resilient material to flow and distribute around the hem line;
prehemming said outer panel;
final hemming said inner and outer panels together by over clinching the peripheral edge of said outer panel;
whereby said resilient material absorbs and distributes the final hemming forces and said outer panel peripheral edge springs back to form a flat hem.
In one embodiment of the invention, applying a resilient material comprises applying the material to the inside surface of the outer panel around the area of the hem line. In another embodiment, applying the resilient material comprises molding a ring of polypropylene around the peripheral edge of the inner panel before nesting the panels together. In yet another embodiment, the step of applying the resilient material comprises applying a hem sealant having resilient properties.
Alternatively, an aluminum wire may be applied around the hem line about the inside surface of the outer panel. The aluminum wire allows the hem pressure to build quicker in the area of the hem during the hemming process, allows the hem to form around the wire and subsequently act as a gap filler in the hemmed closure panel.
The step of final hemming may comprise isolating the bulk of the hemming compression directly on the hem line radius.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.